In general, FSS refers to a plane or surface where uniform patterns are periodically arranged to achieve frequency selective characteristics. Depending on the geometric structure of the uniform pattern, such as shape, size, length, width, etc., of the pattern and electric characteristics of a dielectric, FSS can transmit or cut off a certain frequency band. In FSS, a structure of uniform shape corresponding to a single cycle spatially is generally referred to as a unit cell. Frequency characteristics of FSS vary greatly depending on the shape, geometric structure, and size of an internal pattern of the unit cell, space between unit cells, and electric attributes of other dielectric matters. On the basis of the above principle, diverse methods have been studied to obtain desired frequency characteristics.
The existing FSS has a center connected structure, a loop structure, or other diverse structures. In particular, in order to design a figure geometrically constituting a unit cell to have a maximum length with respect to a given unit area, schemes for preventing, while bending loops in a complicate way, them from being entangled with each other have been proposed. In addition, there are a lot of structures suggested to use as much unit cell space as possible in order to increase space utilization.
Such FSS has the function of separating frequency bands, and therefore, it can be applied to a parabola antenna to accommodate multi frequency bands by one antenna system. The existing antenna systems without FSS can receive only frequencies f1 and f2 by feed horn, but the antenna system with FSS can further accept frequencies f3 and f4 as well as the frequencies f1 and f2.
Generally, FSS unit cells that have been widely used have the shape of rectangle, circle, rectangular loop, circular loop, or the like, and have different frequency response characteristics depending on the shape of each unit cell. However, one problem of the existing FSS is that it can separate frequencies only if the ratio of high frequency band to low frequency band is 1.5 or greater, and it cannot separate frequency bands if the ratio is below 1.5.